loop: fix backing file reference leak on validation error

loop_change_fd() and loop_configure() call loop_check_backing_file()
to validate the new backing file. If validation fails, the reference
acquired by fget() was not dropped, leaking a file reference.

Fix this by calling fput(file) before returning the error.

Cc: stable@vger.kernel.org
Cc: Markus Elfring <Markus.Elfring@web.de>
CC: Yang Erkun <yangerkun@huawei.com>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Yu Kuai <yukuai1@huaweicloud.com>
Fixes: f5c84eff ("loop: Add sanity check for read/write_iter")
Signed-off-by: Li Chen <chenl311@chinatelecom.cn>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Yang Erkun <yangerkun@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>